Series multiple break vacuum arc discharge devices

ABSTRACT

Vacuum arc discharge devices having improved electrode structure are disclosed. Such improved devices comprise a plurality of parallel, electrically conductive paths each having a pair of interrelated arcing gaps disposed therein in a series relationship so as to increase the dielectric strength of such devices and thereby to hold off increased voltages in a cylindrical container whose diameter may be dimensionally limited.

United States Patent [191 Rich [ Mar. 19, 1974 SERIES MULTIPLE BREAK VACUUM ARC DISCHARGE DEVICES Primary Examiner-Herman Karl Saalbach Assistant Examiner-Darwin R. Hostetter [75] Inventor: Joseph Rlch Schenectady Attorney, Agent. or Firm-Jerome C. Squillare: Joseph [73] Assignee: General Electric Company, T. Cohen; Julius J. Zaskalicky Schenectady, NY.

[21] Appl 347071 Vacuum arc discharge devices having improved electrode structure are disclosed. Such improved devices Cl /2 313/231, comprise a plurality of parallel, electrically conductive 313/325, 315/111, 317/62 paths each having a pair of interrelated arcing gaps {51] Int. Cl. H01j 21/20 disposed therein in a series relationship so as to in- Field Of Search 5; crease the dielectric strength of such devices and 315/111; 317/62 thereby to hold off increased voltages in a cylindrical container whose diameter may be dimensionally lim- [56] References Cited ited.

UNITED STATES PATENTS Cl 5 D F 3.679.474 7/1972 Rich 313/217 8 rawmg gums 33 35 3 43 l6 l5 l4 1 9M4 2o 24- j' '1', 1 27- 1' v 1 .m/ 7 A 6 19 A l 13 14 l4 l 1 m f: 5 l2 I 23 5 I F 24 5 I t L 22 39w? B 1 Q 2 l l i 7 14 l c 1 \\\I |7- 35 4 l 33 l8 PATENTEDMAR 1 9 I974 3798.484

saw 2 BF 3 SERIES MULTIPLE BREAK VACUUM ARC DISCHARGE DEVICES The present invention relates to vacuum arc discharge devices operable at high currents without the formation of destructive anode spots therein.

Generally speaking, a vacuum arc discharge device is a circuit protection device comprising at least one pair of spaced electrodes disposed in an evacuated envelope. The device, which'may be in the form of a vacuum switch or a triggerable vacuum gap device, is operable during overload conditions to conduct overload current in an arc across an interelectrode gap between the electrode pair and subsequently to extinguish that arc. Vacuum switches include means for normally conducting current during steady-state conditions without arcing, but are adapted to conduct greatly increased amounts of such current through a conductive plasma across the interelectrode gap during overload conditions. Generally, the plasma is generated by the action of the overload current on the spaced electrodes of the vacuum switch. Triggerable vacuum gap devices do not conduct steady-state current, but are operated only during overload conditions when they are triggered in response thereto. Such triggering causes a conductive plasma to be injected into the interelectrode gap to cause electrical breakdown thereof and to conduct overload current in the arc until the arc is extinguished.

Such vacuum arc discharge devices have been plagued by the problem of anode spots. Anode spots" are areas on the anode which become deteriorated due to the-overload electric current conducted thereto being of an excessively high density at those anode areas. Such overload current is, in large part, caused to concentrate in these areas of the anode by induced magnetic forces present in the interelectrode gap. Other reasons for the concentration of currents in particular anode areas include the fact that the anodes surface area is often relatively small and also that the electrode configuration is characterized by planes and edges at which arcing electric current can localize.

The structure shown in my US. Pat. No. 3,679,474 is a substantial improvement over prior art vacuum arc discharge devices because it makes possible the conduction of arcing currents in excess of 200,000 A (peak) during overload conditions while maintaining the arcing currents at a relatively uniform, but not excessive density over the electrode surface so that the electrode can receive those currents without suffering deterioration. Such a structure, as illustrated in my above-identified patent, incorporated in a vacuum switch, accomplishes the desired result by providing two separate sets of electrodes within an evacuated envelope. A first set of spaced electrodes having a fixed gap anda relatively large surface area, conducts relatively large overload currents. A second set of mating, butt-type electrodes, having a relatively small surface area, conducts smaller, steady-state currents and initiates arcs for conduction of overload currents by the first set of spaced electrodes.

More specifically, the first set of spaced electrodes comprises a pair of primary arc electrode assemblies, which are of opposite polarity. Each of the assemblies includes a plurality of cylindrical electrode members, similarly arranged in a spaced circular array. The assemblies are oriented with respect to each other so that the individual electrode members are alternately interleaved with and extend in opposite directions with respect to the individual electrode members to form a single joint circular array of spaced electrode members of alternating polarity. The adjacent members of this joint circular array are separated by interelectrode spaces or gaps which, during operation of this switch, are arced by overload currents passing between the adjacent, alternating polarity electrodes. The components of magnetic field within the interelectrode gaps transverse to the paths of current conduction across the gaps are substantially reduced due to the opposed direction of the azimuthal magnetic fields surrounding adjacent electrode members. This joint circular array electrode structure, having smooth arcing surfaces, a relatively large total surface area, and a plurality of interelectrode gaps subjected to a minimal magnetic effect on currents arcing thereacross, is thus able to avoid concentration of the arcing currents at particular areas on the electrode members which would otherwise be subjected to high current density and consequent formation of anode spots.

A set of starter electrodes" comprises a pair of mating butt-type electrodes located centrally of the joint circular array, which is adapted, when engaged, to conduct steady-state currents. Additionally, these butttype electrodes, upon separation, can initiate a high current arc which is rapidly transferred by induced magnetic forces to the circular array of alternating, oppositely poled cylindircal electrode members upon which the arc may dwell without the formation of destructive anode spots.

s .n2i 99.s assessed effects fiwth w s fie tic: field on the arc'ng current is achieved by minimizing body force l which is the force per unit volume of conducting fluid at a given point. Such body force is governed by the formula, expressed as a vector product,

wh ere F is the body force, as defined above,

B is the magnetic field existing in the interelectrode ga at the given point, and

is the current density at the same point.

In accord with the general discovery set forth in my aforementioned patent, l eliminate or minim i e the body force by slgastz ntiallyreducing B to an extent that the product J X'B approaches zero and substantially no body force is operative upon current conduction paths between the arc-electrode members. As a result, current conduction path bunching at the anode is avoided and destructive anode spots do not form. Accordingly, devices, such as disclosed in the aforementioned patent, are operative with a relatively simple construction, to permit the carrying of exceedingly large values of current within the device without causing a high current density to exist at any place within the device to cause the formation of destructive anode spots. Thus, the

current threshold for the formation of anode spots is greatly increased and the current carrying capacity of the device is exceedingly high as compared with prior art devices.

While the apparatus described in the aforementioned patent has proved to be a substantial advance over the prior art, a further improvement in its capability is de sirable. The voltage held off by such apparatus is related to the dielectric strength of the interelectrode gaps. Increasing the gap size can increase dielectric strength and thus increase the voltage holdoff capacity. However, the optimum gap dimension between adjacent oppositely poled electrode members is approximately equal to the electrode diameter. Thus, to maintain optimum operating conditions while increasing the gap dimension, requires larger diameter electrode members. To accommodate a sufficient number of spaced electrode members in the envelope with the desired spacing between the respective electrode members, the starter electrodes and the shield member or members, a larger envelope diameter is required. This is an undesirable dimension change because of cost, among other reasons. The cost of such devices generally increases as the square of the diameter. Thus, it is desirable to limit the diameter of the envelopes containing such vacuum arc discharge devices since the cost premium paid for increasing diameter is excessive. Additionally, the problems of fabrication of larger diameter devices are substantially greater as the diameter increases. By the invention herein disclosed, the voltage hold-off capacity of such vacuum are devices is increased without increasing the envelope diameter of the devices.

It is, therefore, an object of the invention to increase the voltage hold-off capacity of vacuum arc discharge devices while simultaneously limiting the diameter of the envelopes enclosing such devices.

It is a further object of the invention, when incorporated in a vacuum arc discharge device of the vacuum switch type, to provide increased voltage hold-off capacity while at the same time facilitating a more rapid separation of the electrode members to establish a current interrupting arc.

It is also an object of the invention, in providing improved voltage hold-off capacity for such a vacuum arc discharge device when utilized as a multiple break vacuum switch, to provide for arc-initiating means.

In accordance with this invention, therefore, an improved vacuum arc discharge device adapted to carry high currents at increased voltage levels without the formation of anode spots is provided. That improved vacuum arc discharge device comprises a hermetically sealed evacuated envelope having a pair of base plates disposed in opposed ends of the envelope. A central base plate is disposed within the envelope intermediate the ends to subdivide the envelope into adjacent first and second regions. Primary arc electrode assemblies are disposed in the respective first and second regions and supported by the respective end base plates proximate those regions. Each of the primary arc electrode assemblies comprises a spaced circular array of cylindrical primary electrode members having smooth cylindrical arcing surfaces, which primary electrode members are oriented generally parallel to each other and normal to the supporting end base plates while extending therefrom into the respective first and second regions. The primary electrode members of each primary arc electrode assembly are substantially coaxially aligned with the primary electrode members of the other assembly. An intermediate arc electrode assembly, supported by the central base plate, comprises a spaced circular array of cylindrical intermediate electrode members also having smooth cylindrical arcing surfaces. The intermediate electrode members are oriented generally parallel to each other and normal to the central base plate while extending into both the first and second regions in an interleaved alternating sequence with the primary electrode members disposed in those respective regions to thus form, in each region, a joint circular array of spaced-apart, oppositely extending electrode members which are oriented parallel to each other. Adjacent ones of the oppositely extending electrode members in each joint circular arrgy aitemate in polarity to cause the vector product J X B to be insignificantly small in the spaces between the adjacent electrode members in the joint circular array, where J equals current density of the arc discharges between any givgn pair of oppositely extending electrode members and B equals the magnetic field between any given pair of electrode members due to the current paths within the electrode members when the device is in a current conduction condition. Means are also provided for causing an electric arc breakdown to be established between the adjacent electrode members of the joint circular array and means are also provided for connecting the primary arc electrode assemblies in circuit with an electric load.

The novel features believed characteristic of the present invention are set forth in the appended claims. The invention itself, however, together with further objects and advantages thereof, can best be understood with reference to the following detailed description, taken in connection with the accompanying drawings, in which:

FIG. 1 is a vertical cross-sectional view of a vacuum arc discharge device constructed in accord with one embodiment of the present invention, with the bulk of the primary and intermediate electrode members being omitted for purposes of clarity.

FIG. 2 is a horizontal sectional view taken along the lines 22 of FIG. 1 with the electrode members omitted in FIG. 1 being included to show their relationship.

FIG. 3 is a vertical cross-sectional view, with parts broken away, of a triggerable vacuum gap device constructed in accord with another emodiment of the present invention;

FIG. 4 is a schematic representation of one of the parallel current conduction paths, through adjacent electrode members of the device of FIGS. 1, 2, and 3, and

FIG. 5 is a vertical cross-sectional view of an alternative structure to that illustrated in FIG. 3.

A multiple break vacuum switch 10 constructed in accord with the present invention, is illustrated in FIG. 1. As shown therein, the vacuum switch 10 includes an hermetically sealed, evacuated envelope 11 defined by a pair of insulating sidewall members 12 which are hermetically attached to opposed sides of a central base plate 13 by vacuum seals as shown at 14. The sidewalls 12 extend in opposite directions from the central base plate member 13 to form a pair of similar, defined regions, A and B, respectively. A pair of conductive base plates 15 and 17 defining the ends of envelope 11 are oriented parallel to the central base plate 13 and hermetically attached to the respective sidewall members 12 to enclose regions A and B.

A plurality of electrically conductive parallel paths, each defined by three staggered cylindrical electrode members, are provided for conducting overload current through switch 10. Specifically, a pair of similar, primary arc-electrode assemblies, comprising circular arrays of cylindrical, primary electrode members 16 and 18, respectively, are similarly disposed in the respective envelope regions A and B and supported by respective end base plates and 17. Each of those primary electrode members 16 and 18 is characterized by a smooth cylindrical arcing surface. Within their respective assemblies, the primary electrode members 16 and 18 are oriented normal to their respective supporting base plates and parallel to each other so that the individual primary electrode members 16 are coaxially aligned with respect to corresponding primary electrode members 18.

An intermediate arc-electrode assembly is supported by the central base plate 13. The intermediate electrode assembly comprises a spaced circular array of cylindrical, intermediate electrode members 19 which have smooth cylindrical arcing surfaces. These intermediate electrode members 19 oriented parallel to each other and normal to the central base plate 13, include opposite end portions which extend into each of the envelope regions A and B to interleave in alternating sequence with the circular arrays of primary electrode members 16 and 18 respectively. In each region, a joint circular array is thus formed comprising a plurality of oppositely extending electrode members of alternating polarity.

In FIG. 2, a horizontal cross-sectional view of section B of switch 10 illustrates the above-described alternating array of members 18 and 19 wherein members 19 are shown in section and the tops of members 18 are viewed as solid members. As may be seen from FIG. 2, each pair of members 18 and 19 define an interelectrode gap 21 therebetween to which the are initially struck by the starter electrodes 22 and 23 is transferred for are extinction to achieve current interrupt ion. Current paths .25 established within gaps 21 each carry a portion of the total current being conducted through device 10. The joint circular arrays in each region A and B, together comprise a plurality of parallel, electrically conductive paths through envelope [1. Each parallel electrically conductive path as defined by a pair of oppositely extending primary electrode members 16 and 18 and an intermediate electrode member 19 immediately adjacent and forming respective arcing gaps with both sets of primary electrode members, thus includes a pair of interelectrode gaps 21 provided in a series relationship so that the voltage drop through such a path is cumulative of the voltage drops across each of those gaps. Thus, voltage hold-off capacity is doubled by disposing a pair of similar interelectrode gaps 21 in series in each of the parallel, conductive paths. This may resultin a lengthening of the envelope dimension, but the envelope diameter can be retained unchanged.

The present invention thus has decided advantages over an alternative approach for increasing voltage hold-off capacity whereby the size of a single interelectrode gap, disposed in each parallel path, is increased in size. For example, doubling the size of such a single gap can require a consequent increase in both the electrode diameter and envelope diameter. Moreover, such a single gap approach may not produce the desired proportional increase in voltage hold-off capacity.

The electrode members are made of a material having a sufficiently high vapor pressure which, upon the presence of the high magnitude current arc, provides a copious quantity of metallic particles during arcing for supplying conduction carriers during the operation of the device.

To prevent short-circuiting of the device during operation, it is essential that the metallic particles not be allowed to settle on the interior surface of the insulating sidewall members 12. Therefore, a pair of shield members 26 supported from members 27 embedded in the insulating sidewall members 12 are disposed within the evacuated regions A and B between the insulating members and the electrodes to shield the electrodes from metallic particles expelled from the electrode members.

Arc-initiating means are provided in the form of starter arc electrode contacts which comprise two pairs of mating, but separable, massive butt-type contacts 22 and 23 disposed serially along a single conductive path which conducts steady-state currents. Such a vacuum switch is called a multiple break switch because the circuit in which the switch is disposed is simultaneously broken at a plurality of places along the steady-state current conduction path by the respective mating contact pairs. These butt-type mating contact pairs, centrally disposed within the respective joint circular arrays, are separable to define starter arcing gaps 24 in each of the regions A and B. Upon current overload, the butt-type contacts can thus be separated to generate arcs, which are transferred to the joint circular arrays for conduction and subsequent extinction.

The contacts 22 are supported by conductive actuating rods 33, which rods are reciprocally movable through the end base plates by means of bellows 34. These bellows are hermetically attached to the rods and the end base plates and are shielded from settling metal particles by shields 35. A conductive rod 36, fixedly supported by the central base plate 35, extends into the regions A and B and mounts butt contacts 23 at its opposed ends. During steady-state conditions the mating pairs of butt-type contacts are abutting in closed circuit position and the steady-state current path is through the rod structure comprising rods 33 and 36 and contacts 22 and 23. Current so conducted generates forces which tend to separate the mating butt-type contacts. It is therefore desirable to be able to exert resisting forces to keep the butt-type contacts engaged until such time as it is desired to release them. However, when substantial currents are conducted at the initiation of current overload conditions, the forces generated are substantial and the resisting forces must be of similar magnitudes. The switch structure must be able to accommodate such forces. Thus, the arcinitiating electrodes are so arranged that upon mating the movable contacts 22 can exert equal and opposite forces on the stationary contacts 23. Therefore, the supporting central base plate 13 and associated vacuum seals 14, which connect base plate 13 to the insulating sidewall members 12, will not be unduly stressed.

An additional advantage to vacuum switches in accord with the invention over single break switches, utilizing a single pair of starter electrode contacts, is found in the use of two pairs of such contacts. The use of two simultaneously-operable series connected gaps results in the achieving of a sufficiently high voltage drop across the starter gaps 24 to effectuate transfer of the arc to the rod electrode structure of members 16,

18 and 19 sooner and thereby achieve more rapid switching.

In operation of vacuum switches in accord with the invention, during steady-state current conditions, current is conducted centrally through the vacuum switch along the path defined by rods 33 adn 36 which are electrically connected by the mating electrode pairs 22 and 23. When an overload condition occurs, the mating ones of each pair of butt electrodes are separated by actuating rods 33 and arcs are struck in gaps 24. By withdrawing electrodes 22 a sufficient distance from electrodes 23, gaps 24 are made sufficiently large so that induced magnetic forces can act upon the struck arcs, thereby causing them to spread out almost instantaneously into the plurality of gaps 21 between the alternating polarity, adjacent electrode members in each joint circular array. The overload current then being conducted to the switch is conducted through heavy leads 43, 44 symmetrically connected to respective end base plates and 17. Such overload current may then pass directly to the primary electrode members supported thereby and then through the joint circular arrays of electrode members in regions A and B to the other end base plate and out of the device through similar heavy leads symmetrically connected thereto. The plurality of now formed high current arcs, sustained by the overload current now passing through the joint circular arrays, is sustained by a conductive plasma, comprising metallic particles from the electrodes in each joint circular array.

Conduction between a single such gap pair and the broad dispersion thereof is illustrated schematically in FIG. 4. This plasma permits the arcs to be transferred across the pair of gaps 21 in each parallel conductive path until the value of arcing current passes through a zero value and conduction ceases, giving the specie of the plasma an opportunity to cool and condense upon a relatively cool surface, as for example the shield or the members l6, l8 and 19 of the rod array. Whenthe next cycle of alternating voltage is applied across the open contact electrodes, the high dielectric strength of the vacuum within the device prevents reestablishent of current.

When it is desired to close the mating butting contacts to resume steady-state current conduction, the forces developed upon mating of the butt electrode pairs are equally applied at opposed ends of the supporting rod 36 so that no undue stress is applied to the envelope structure.

FIG. 3 illustrates a triggerable vacuum gap device 110 which incorporates the basic concept of the invention by placing in a series relationship in each of a plurality of individual conduction paths in the device, a pair of vacuum gaps so as to increase the voltage holdoff capacity.

The structure of the triggerable vacuum gap device 110 is similar to that of the vacuum switch 10 except that there are no butt-type starter electrode contacts to operate during steady-state conditions, since this device is purely an overload responsive mechanism. The electrode structure comprises a pair of primary arc electrode assemblies having primary electrode members 116, 118 and an intermediate arc electrode assembly having intermediate electrode members 119 arranged to form a pair ofjoint circular arrays (similar to those of the vacuum switch, shown in FIGS. 1 and 2) which define a plurality of parallel conductive paths through the envelope 111 of this device. A plurality of gaps is disposed in each of the parallel conductive paths.

To trigger the triggerable vacuum gap device to a conductive state requires triggering electrode assembly means shown at 141 and 142, well known in the prior art and shown, for example, in U.S. Pat. No. 3,465,l92 and 3,465,206. The triggering means electrode assembly injects a cloud of an electron-ion plasma into regions A, B in the interelectrode gaps between the electrode members in the primary and intermediate arc electrode assemblies. An aperture 140 is disposed in central base plate 113 so that one trigger electrode assembly can inject plasma into both regions A and B. Normally, however, trigger electrode assemblies are placed in each of regions A and B. Such triggers are energized directly or indirectly by overload voltages applied to the lines which device is connected with or designed to protect. Overload currents conducted from the load circuit to the electrode members of this device will, because of the presence of the injected conductive plasma, instantaneously be short-circuited through a high current are in the interelectrode gaps between adjacent electrode members, traversing a pair of gaps along each conductive path, each of which establishes its own voltage drop into the arc circuit.

The electrode structure arrangement of this invention is advantageous because the voltage hold-off capacity of a vacuum arc discharge device can be doubled without increasing the diameter of the device. This is possible because the total dielectric strength in each parallel conductive path can be doubled without increasing the diameter of each electrode member in the structure. Moreover, in a double break vacuum switch incorporating this invention, the life of flexible bellows between the actuating rods 33 and the respective end plates 15 and 17 is considerably prolonged over the life of similar bellows in a single break device designed to handle the same current.

FIG. 5 of the drawing illustrates an alternative embodiment of the device of FIG. 3 wherein like numbers are identified by like numerals. In FIG. 5, centerplate 113 is made thick enough to accommodate a transversely-disposed trigger electrode assembly 142 in lieu of the two trigger electrode assemblies utilized in the device of FIG. 3. The use of a single centrally disposed trigger'assembly provides for more reliable simultaneous instantaneous breakdown of the respective gaps in both arcing chambers A and B without the necessity of utilizing any external means to insure simultaneous energization of two separate trigger assemblies.

Wile I have shown and described various embodiments of the invention, it will be obvious to those skilled in the art that changes and modifications may be made without departing from the invention. Thus, for example, although the invention has been shown in an embodiment having two serially connected sections A and B, greater numbers may be serially juxtaposed, particularly in the triggerable gap embodiment. It is therefore intended in the appended claims to cover all such changes and modifications that fall within the true spirit and scope of this invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. An improved vacuum arc discharge device for carrying high currents at increased voltage levels without the formation of anode spots, comprising:

a. an hermetically sealed evacuated envelope;

b. first and second end base plates disposed in opposed ends of said envelope;

c. a third base plate disposed in said envelope intermediate the ends thereof to subdivide said envelope into adjacent first and second regions;

d. first and second primary arc-electrode assemblies disposed in respective ones of said first and second regions and supported by said respective first and second end base plates;

e. each of said primary arc-electrode assemblies comprising a spaced circular array of cylindrical primary electrode members having smooth cylindrical arcing surfaces, said primary electrode members being oriented generally parallel to each other and normal to their respective supporting base plate and extending therefrom into respective ones of said adjacent first and second regions, said primary electrode members of said first primary arcelectrode assembly being substantially coaxially aligned with said primary electrode members of said second primary arc-electrode assembly;

1. an intermediate arc-electrode assembly supported by said third base plate;

g. said intermediate arc-electrode assembly comprising a spaced circular array of cylindrical intermediate electrode members having smooth cylindrical arcing surfaces, said intermediate electrode mem-' bers being oriented generally parallel to each other and normal to said third base plate while extendig therefrom into both said first and second regions, said intermediate electrode interleaved in alternating sequence with said primary electrode members to form, in each region, a joint circular array of spaced-apart, oppositely extending electrode members oriented parallel to each other;

h. adjacent ones of said oppositely extending electrode members in each said joint circular array alternating in polarity, thus causing the vector product J X B to be insignificantly small in the spaces between said adjacent electrode members in said joint circular array where A J =current density in the arc discharges between any given pair of oppositely extending electrode members B=magnetic field between any given pair of electrode me'r'nber s due to current paths'wi'thin said gerable vacuum gap device and said means for causing an electric arc break-down in each said joint circular array comprises a trigger electrode assembly for supplying electron ion plasma.

3. The device of claim 1 wherein the device is a vacuum switch and said means for causing an electric arc break-down in said joint circular array comprises normally closed separable starter electrode contacts.

4. The device of claim 3 wherein said starter electrode contacts constitute respective pairs of mating,

butt-type contacts disposed centrally of said respective joint circular arrays.

5. The device of claim 2 wherein a separate trigger gap assembly is associated with each of said first and second regions.

6. The device of claim 2 wherein a single trigger gap assembly is located intermediate said first and second regions and adapted to simultaneously inject electron ion plasma into said regions when actuated.

7. The device of claim 3 wherein one pair of butttype starter electrode contacts is located within each of said first and second regions and adapted to be actuated from a circuit closed to a circuit open condition simultaneously.

8. The device of claim 7 wherein said contacts are of sufficient current carrying capacity to carry the normal rated current of said device during non-interrupting operation thereof. 

1. An improved vacuum arc discharge device for carrying high currents at increased voltage levels without the formation of anode spots, comprising: a. an hermetically sealed evacuated envelope; b. first and second end base plates disposed in opposed ends of said envelope; c. a third base plate disposed in said envelope intermediate the ends thereof to subdivide said envelope into adjacent first and second regions; d. first and second primary arc-electrode assemblies disposed in respective ones of said first and second regions and supported by said respective first and second end base plates; e. each of said primary arc-electrode assemblies comprising a spaced circular array of cylindrical primary electrode members having smooth cylindrical arcing surfaces, said primary electrode members being oriented generally parallel to each other and normal to their respective supporting base plate and extending therefrom into respective ones of said adjacent first and second regions, said primary electrode members of said first primary arc-electrode assembly being substantially coaxially aligned with said primary electrode members of said second primary arc-electrode assembly; f. an intermediate arc-electrode assembly supported by said third base plate; g. said intermediate arc-electrode assembly comprising a spaced circular array of cylindrical intermediate electrode members having smooth cylindrical arcing surfaces, said intermediate electrode members being oriented generally parallel to each other and normal to said third base plate while extendig therefrom into both said first and second regions, said intermediate electrode interleaved in alternating sequence with said primary electrode members to form, in each region, a joint circular array of spaced-apart, oppositely extending electrode members oriented parallel to each other; h. adjacent ones of said oppositely extending electrode members in each said joint circular array alternating in polarity, thus causing the vector product J X B to be insignificantly small in the spaces between said adjacent electrode members in said joint circular array where J current density in the arc discharges between any given pair of oppositely extending electrode members and B magnetic field between any given pair of electrode members due to current paths within said electrode members when said device is in a current conduction condition; i. means for causing an electric arc break-down to be established between said adjacent electrode members in each said joint circular array; and j. means for connecting said primary arc-electrode assemblies in circuit with an electric load.
 2. The device of claim 1 wherein said device is a triggerable vacuum gap device and said means for causing an electric arc break-down in each said joint circular array comprises a trigger electrode assembly for supplying electron ion plasma.
 3. The device of claim 1 wherein the device is a vacuum switch and said means for causing an electric arc break-down in said joint circular array comprises normally closed separable starter electrode contacts.
 4. The device of claim 3 wherein said starter electrode contacts constitute respective pairs of mating, butt-type contacts disposed centrally of said respective joint circular arrays.
 5. The device of claim 2 wherein a separate trigger gap assembly is associated with each of said first and second regions.
 6. The device of claim 2 wherein a single trigger gap assembly is located intermediate said first and second regions and adapted to simultaneously inject electron ion plasma into said regions when actuated.
 7. The device of claim 3 wherein one pair of butt-type starter electrode contacts is located wIthin each of said first and second regions and adapted to be actuated from a circuit closed to a circuit open condition simultaneously.
 8. The device of claim 7 wherein said contacts are of sufficient current carrying capacity to carry the normal rated current of said device during non-interrupting operation thereof. 